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Wednesday, May 16, 2012

Thermodynamics

Thermodynamics is the branch of science which deals
with energy (heat) transfer and its effect on state or condition of a system.
Essentially thermodynamics pertains to the study of:

1.Interaction of system & surrounding:-

It
relates the changes which the system undergoes to the influences to which it is
put.

2.Energy & its transformation:-

Energy inter-conversion in the form
of heat & work. James Joule had proved with his well known experiment that mechanical
work can be converted into heat energy. The credit for using heat by converting into work goes to James Watt who produced the first "Steam Engine"
& paved the way for industrial revolution.

3.Relationship between heat, work & physical
properties.

Such as pressure, volume & temperature of the
working substance (water for steam engine, petrol for bike etc) employed to
obtain energy conversion.

Thermodynamics has been excellently defined as the
science of three "Es" namely Energy, Entropy & Equilibrium. The
principles & concepts of thermodynamics are important tools in the
innovation, design, development & improvement of engineering processes,
equipment & devices which deals with effective utilization of energy.

The applications of engineering thermodynamics in the field
of energy technology are as follows:-

3.Chemical process plants & direct energy
conversion devices. A large number of processes in various fields such as
agriculture, textiles, dairy, drugs & pharmaceutical industries are also
governed by thermodynamics principle.

Macroscopic & Microscopic approach.

or Classical & Statistical Thermodynamics.

There are two approaches for investigating the
behavior of a system.

Macroscopic approach.

It is also known as
classical thermodynamics which is concerned with gross or overall behavior of
matter. In classical thermodynamics ,the
analysis of thermodynamics systems is
explained with the measurable property like pressure ,volume, temperature etc. It doesn’t explain the structure of
matter.(i.e No attention is focused on the behavior of individual particles constituting the matter.)The volume considered
is very large compared to molecular dimension & the system is regarded as
bulk (whole/single).The study is made of overall effect of several molecules.
The behavior & activities of the molecules are averaged. Only a few
variables are needed to describe the state or condition of matter. The
properties like pressure, temperature etc. needed to describe the system can be
easily measured & felt by our
senses. The particles of system required simple mathematical formula for
analyzing the system for e.g. Piston cylinder assembly of an IC engine, volume
occupied by the gas for each position of piston etc. It is based on the concept
of continuum. The macroscopic form of energy are those a system possess as a whole
with respect to some outside reference frame, such as potential and kinetic
energies.

Microscopic point of view

This is also known as statistical thermodynamics and is
directly concerned with structure of matter. It focuses on statistical behavior
of mass consisting numerous individual molecules and correlates macroscopic
properties of the matter with molecular configuration and intermolecular
forces. It explains the structure of matter. Few coordinates are not sufficient
to describe this system. Concept of
continuum is not valid for this viewpoint. The microscopic form of energy are
those related to the molecular structure of a system and the degree of
molecular activity, and they are independent of outside reference frames. The sum
of all the microscopic forms of energy is called the internal energy of a
system.

Thermodynamics system:-

It is defines as a definite area or a space where some
thermodynamic process takes place. It is a region where thermodynamic process are
studied. A thermodynamic system has it’s boundary and anything outside the boundaries is called it’s
surrounding. These boundaries may be fixed like that of a tank enclosing a
certain mass of compressed gas, or movable like boundary of a certain volume of
liquid in a pipe line.

Thermodynamic system
represents a prescribed & fixed quantity of matter under consideration to
analyze a problem; to study the changes in its properties due to exchange of
energy in the form of heat & work. The system may be quantity of steam, a mixture
of vapor & gas or a piston cylinder assembly of an IC engine & its
contents.

For the description of thermodynamic system some of
the following quantities need to be specified.

1.Quantity as well as composition of matter.

2.Measurable properties such as pressure,
temperature & volume of the system

3.Energy of the system

The combination of matter & space, external to
the system that may be influenced by the changes in the system is called
surrounding or environment. The thermodynamic system & surrounding are separated
by an envelope called boundary of the system. The boundary represents the limit
of the system and may be either real or imaginary and may change shape, volume,
position or orientation relative to observer for e.g. an elastic balloon which
is initially spherical in shape may change into cylindrical shape or some other
geometrical shape or may get squeezed to reduce volume during a certain period.
As such, the boundary of the gas in the balloon couldnot retains the same size
and shape. Further, the boundary may be diathermal or adiabatic depending upon
whether it allows or not exchange of energy in the form of heat. The walls of
the boundary which does not let heat transfer to take place across them are
named as adiabatic. In contrast the walls that do allow heat interaction across
them are called diathermic. The diathermic system can be classified into.

A.Closed system

A close system can change energy in the form of heat & work with
its environment but there is no mass transfer across the system boundary. The
mass within the system remains the same and constant, though its volume can
change against flexible boundary further the physical nature & chemical
composition of the mass may change. Thus, a liquid may evaporate, a gas may
condense or a chemical reaction may occur between two or more components of the
system for e.g.

Cylinder fitted
with movable piston.

1.Motor car battery.

2.Pressure cooker

3.Refrigerator

4.Ice-cream greaser

5.Bomb Calorimeter etc.

B.Open system:

A open system has mass exchanged with the surrounding along with
transfer of energy in the form of heat and work. The mass within the system
doesn't necessarily remain constant. It may change depending upon mass inflow
& mass outflow for e.g.

1.Water wheel.

2.Motorcar engine:-

The engine initially draws charge
(mixture of air and petrol) and finally exhausts the spent up gases to the
surrounding atmosphere. The mass flows across the engine boundary.

3.Steam generator (Boiler):-

A boiler is a device which converts
water from liquid vapor freeze. Here consist of system change, water froze into
and steam froze out of the system.

Most of the
engineering devices are open system.

C.Adiabatic system:

There exists wall or boundary
which doesn't allow heat transfer to take place across them, no matter how not
one member is compared to other. Such boundaries are named as adiabatic. An
adiabatic system is thermally insulated from its environments. It is enclosed
by adiabatic walls and can exchange energy in the form of work only for e.g. a
pipe carrying heat enclosed by thermal insulation.

D.Isolated
system:

An isolated system is a
fixed mass and energy; it exchanges neither mass nor energy with another system
or with surrounding. An isolated system has no interaction with the
surrounding, it neither influences the surrounding, nor it is influenced by it.
When a system and its surrounding are taken together they constitute an
isolated system. The universe can be considered as an isolated system & so
is the fluid enclosed in the perfectly insulated closed vessel (thermos/flask)

E.Homogeneous & heterogeneous:

A system consisting of single phase is called homogeneous
system for e.g.

1.Ice, water, dry saturated steam.

2.A mixture of air and water vapor

3.Mixture of ammonia in water

A system whose mass
content is non uniform through i.e. it consists of more than one phase is
called heterogeneous system for e.g.

1.A mixture of ice and water

2.A mixture of water & gasoline

3.A mixture of water & mercury

Thermodynamic property

Thermodynamic
property refer to the characteristic which can be used to describe the
condition or state of the system for e.g. temperature, pressure, chemical
composition, color, volume, energy etc. The salient aspects of thermodynamic
properties are:-

a)It is a measurable characteristic describing
system & helps to distinguish one system to another

b)It has a definite unit value when the system is
in a particular state

c)It is dependent only on the state of the system,
it does not depend on path or route. The system follows to attain the
particular state.

d)It's differential is exact

If 'P' is the
thermodynamic property with dP representing its differential change, then the
integral between initial state 1 to final state 2 of system will have only one
value given by 'O2'

Since, the thermodynamic
property is the state of a system; it is referred to as point a function or
state function. There are two kinds of thermodynamic property namely Intensive
and extensive.

a)Intensive Property:

It is independent of the extent or the mass
of the system. Its value remains same whether consider whole the system or part
on it for e.g. pressure, temperature, density, viscosity, composition, thermal
conductivity, electrical potential etc.

b) Extensive property:

It depends on the mass or
extent of the system. Its value depend on how big portion of the system is
being considered for e.g. energy, enthalpy (heat contained), entropy (disorder),
volume etc.

Specific property:-

An extensive property expressed per unit mass of the
system is known as specific property for e.g. specific volume, specific energy,
specific entropy, specific enthalpy etc.

Equilibrium:-

Equilibrium is the concept associated with the
concept of tendency for spontaneous change in the value of any macroscopic
property of the system when it is isolated from its surrounding.

1.
Mechanical equilibrium (Equality of pressure)

Condition or state in which
there is no unbalance force within the system and nor its boundaries.
Mechanical equilibrium implies uniformity of pressure i.e. there is one value
of pressure for the entire system; diffusion takes place to wipe out the
unbalance and attain the state of mechanical equilibrium.

2.Chemical Equilibrium:-

A system
in chemical equilibrium may undergo a spontaneous change of internal structure
due to chemical reaction or diffusion (transfer of matter) for e.g. there will
be spontaneous change in the properties of the mixture of oxygen & gasoline
once it is ignited. Chemical equilibrium represents that condition a state of
the system when all chemical reaction in it have ceased & there is no mass
diffusion.

3.Thermal Equilibrium

A system is said to be in thermal equilibrium, when is no
temperature difference between the parts of the system or between the system
and surroundings.

4.Thermodynamic Equilibrium

A system
which is simultaneously in a state of mechanical equilibrium, chemical
equilibrium & thermal equilibrium is said to be thermodynamic equilibrium.

⋆ State, path, process cycle.

Consider a system constituting by a
gas enclosed in a piston cylinder assembly of reciprocating machine.
Corresponding to the position of the piston at any instant, the condition of
the system will be prescribed by pressure, volume & temperature of the gas.
When all such properties have a definite value, the system is said to be exist
at definite state. State is thus condition of the system identified by
thermodynamic property.

When a piston move outwards the properties of the system change
(pressure decreases, volume increases). Any such operation in which properties
of the system change is called change of state. The locus of series of states
through which a system passes is going from initial state to final state
constitutes the path.

A complete specification of the
path is referred to as process.

When
a system in a given state undergoes series of processes such that the final
state is identical with initial state, a cyclic process or a cycle is said to
be be executed. The cycle 1-A-2-B-1 consist of 1-A-2 & 2-B-1(process),
cycle 1-C-2-B-1 consist of 1-C-2 & 2-b-1 (process).

Quasi-state or Quasi Equilibrium process

Some unbalanced
potential must exist either within the system or between a system & its
surrounding to promote the change of state during a thermodynamic process.
Consider a system of the gas in cylinder fitted with piston upon which are placed many small pieces of
weights.

The
upward force exerted by the gas just balances the weight on the piston. The
system is initially in equilibrium in equilibrium state indentified by pressure
P1, Volume V1 & Temperature T1. When these
weights are removed slowly one at a time, the unbalanced potential is
infinitesimally small. The piston will slowly move upward & at particular
instant of piston travel, the system would be almost closed to the state of
equilibrium. The departure of system
from thermodynamic equilibrium state will be infinitesimal small. Every state
passed by the system will be in equilibrium state. The locus of series of such equilibrium state
is called Quasi-static or Quasi Equilibrium process.

Thus, when the process is carried out in such a way that
at every instant, the system deviation from the thermodynamic equilibrium is
infinitesimal, then the process is known as quasi-static or quasi equilibrium
process. It is a very slow process and each state in the process may be
considered as an equilibrium state. It is also known as reversible process.

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